Field of the Invention
This invention relates to automatic and manual/automatic transmissions and more particularly to motor vehicles including such transmissions and devices for detecting and preventing undesirable operating conditions during vehicle operation as a result of critical equipment failures.
The present invention relates to control systems and methods of operation for manual/automatic heavy duty transmission systems wherein gear ratio selection and gear shift timing decisions are made and/or executed based upon measured and calculated parameters such as engagement condition of the transmission, vehicle or transmission output shaft speed, transmission input shaft speed, engine speed, throttle position, and the like. Until recently, transmissions, particularly for heavy duty applications, have historically been the manual shift type requiring actuation of a clutch and gear shift lever by the driver in order to change final drive gear ratios. Gear ratio selection is accomplished using a shift lever coupled to the transmission for mechanically disengaging and engaging various gear ratio operation states of the transmission. Due to the large number of gear ratio operation states associated with compound transmissions or transmissions having a splitter type auxiliary section, a great deal of effort is required to manually shift throughout the entire set of available gear ratio operation states of the transmission. More recently, successful devices which combine manual and automatic shifting operation under certain conditions have been developed such as the invention disclosed in U.S. Pat. No. 4,722,248, the disclosure of which is incorporated herein by reference.
When an electronic shift control system is coupled to a manual/automatic transmission, a lessening of operator fatigue and improved fuel economy result. A device which combines manual shifting of low vehicle speed gear ratios with automatic shifting between sequentially related higher vehicle speed gear ratios, and more particularly between the "top two" available gear ratios of the manual/automatic transmission, reduces driver fatigue. By providing for automatic gear selection of the two highest gear ratio operation states of the transmission, or the "top two" gears, an automatic shift schedule can be developed for increasing fuel economy. Additionally, an automatic gear shift device can be used in conjunction with cruise control electronic systems compatible with heavy duty truck applications.
One special attention situation that arises, in the case of a manual/automatic transmission controlled by an electronic controller including cruise control algorithms and algorithms for shifting the "top two" gears, is sensor failure. When the "top gear" switch, or the electronic sensing device that indicates the driver is requesting automatic shift mode fails, special condition operating conditions may exist or arise. More particularly, the "top gear" switch may fail closed indicating that the driver is requesting automatic gearshift operation when in actuality the switch is open or the switch is in the open position according to the actuator lever, yet the contacts remain fused together. Another switch failure state occurs when the switch contacts will not close, thus providing a stimulus to the electronic controller which responds and disengages automatically engaged auto-shift gears thereby allowing a vehicle to remain in neutral and increase in speed without engaging one of the auto-shift selectable transmission gears to enable engine-drag braking of the vehicle. When the "top gear" switch fails open, the auto-shift mode cannot be enabled. Thus, an undesirable operating condition arises when a vehicle is traveling downhill at the time and the transmission remains in neutral after the driver has requested automatic gear selection operation.
Another undesirable state occurs when the switch fails closed, i.e., auto-shift mode is requested at all times, and the transmission remains in one of the top two gears even though the vehicle slows down below a predetermined speed. This condition may occur when the vehicle is coming to a stop or encounters an incline and the vehicle slows as a result of the incline. Lower gear selection becomes impossible if the switch is failed closed, and the control system will not disengage auto-shift mode gear engagement, and thus the vehicle engine may stall due to improper gear ratio selection. An additional issue exists when the driver intentionally allows the vehicle to coast downhill in neutral to achieve higher vehicle speeds than the normal road speed governor of the vehicle engine will allow.
A control system for controlling a cruise control system and an associated heavy duty truck transmission having at least two automatically selectable gear ratio operational states is needed which includes algorithms for avoiding the conditions which exist due to component failure or certain driver practices that create undesirable operating conditions.